In closed cycle refrigerating systems intended to provide temperatures in the usual household or commercial range, the refrigerant gas is compressed and then condensed, the condensed fluid is throttled and evaporated to produce the refrigerating effect, and the evaporated gas is returned to the compressor to complete the cycle. The refrigerants are typically Freon-type pure gases, and a simple single stage reciprocating or rolling piston compressor is sufficient to achieve the modest pressures and efficiencies required.
However, where the refrigerating system is intended to provide very low temperatures in the cryogenic range, such as between 65 degrees and 150 degrees Kelvin, the refrigerants comprise cryogenic gases, usually having boiling temperatures below 130 degrees K, such as Nitrogen, which has a normal boiling temperature of 77 degrees K, or Argon, which has a normal boiling temperature of 87 degrees K, or Methane, which has a normal boiling temperature of 112 degrees K. These cryogenic gases have typically required the use of very high pressure gas systems involving specially designed multistage compressors or high pressure oil-less compressors. Such systems are expensive to manufacture and operate and require frequent maintenance.
Various expedients have been used in closed cycle refrigerating systems operating in the intermediate range between the household refrigerating temperatures and about 150 degrees K to produce refrigeration at pressures low enough that a single stage oil-lubricated compressor designed for higher temperatures can still be used. For example, mixtures of primarily Freon-based refrigerants have generally been used rather than pure Freon refrigerants to permit lower pressures. Such mixed-gas refrigerants have also been used with cascaded heat exchangers or with successive stages of vapor-liquid separation in order to permit use of a single compressor for the system. Such expedients are well described, for example, in U.S. Pat. 3,768,273, issued Oct. 30, 1973 to Missimer.
However, for temperatures in the range of 65 degrees K to 150 degrees K, where very low boiling point cryogenic gases such as Nitrogen, Argon or Methane are involved, the required ratios between the low input pressures and the high discharge pressures for refrigerators operating in a normal ambient environment are so great that only multistage compressors have heretofore been used. The number of additional heat exchangers or intermediate phase separators becomes so great as to be deemed impractical.